1. Field of the Invention
The present invention relates to a power controlled electrical circuit protection device. More particularly, the present invention provides methods and apparatus for simultaneous control of several low or medium power loads while providing high voltage isolation to the power control element and overcurrent protection of the loads.
2. Introduction to the Invention
Many diverse applications exist for delivering low to moderate power simultaneously to a plurality of loads. The loads may be indicators, sensors or actuators such as small electric motors or solenoids for converting electrical energy into motive force. In these applications the power/data/control levels may be provided via a multi-line bus from a central control point, such as a computer or computer-based controller. Good engineering practice dictates that each load needs to be protected from short/overload fault conditions.
Conventional protection methods include metal-link fuses, circuit breakers, and polymeric positive temperature coefficient (xe2x80x9cPPTCxe2x80x9d) resistor devices, for example. When an overcurrent condition occurs, fuse links rupture irreversibly, breakers trip and must be manually reset, and PPTC devices switch or trip from a few ohms low resistance to a high impedance state in excess of a Mohm high resistance. Switching occurs as a result of local heating of the PPTC device. Unique to PPTC devices, when the overcurrent condition is corrected and the power to the circuit is removed, the PPTC device cools and automatically resets itself to its low resistance state. This automatic reset function provides a significant advantage over metal-link fuses, which must be manually removed and replaced at a fuse block, or breakers that must be manually reset, by a technician or other user of the apparatus, device or appliance being protected.
It is known to form an array of thin film or thick film resistors upon a nonconductive ceramic substrate and package the array in a standard configuration such as dual inline package (DIP) or single inline package (SIP). While such devices are widely used to terminate computer buses with fixed impedance loads, they are entirely passive and do not provide overcurrent protection in a manner equivalent to metal-link fuses, breakers or PPTC devices.
It is also known to provide a single integrated circuit device integrally including load termination resistors and series switches formed of field effect transistor (FET) devices. The active FET devices enable the resistor loads to be collectively disconnected in response to a control signal. Examples of such devices are found in U.S. Pat. No. 5,381,034 issued on Jan. 10, 1995 to Thrower et al., entitled xe2x80x9cSCSI Terminatorxe2x80x9d, and U.S. Pat. No. 5,382,841 issued on Jan. 17, 1995 to Feldbaumer, entitled xe2x80x9cSwitchable Active Bus Termination Circuitxe2x80x9d, the disclosures thereof being incorporated herein by reference thereto. In these patents, resistors are formed and trimmed as part of an integrated circuit fabrication process, but the resistors, while providing desired bus load impedances, do not provide self-resetting protection fuses in the manner provided by PPTC devices.
It is further known to couple a positive temperature coefficient (xe2x80x9cPTCxe2x80x9d) or negative temperature coefficient (xe2x80x9cNTCxe2x80x9d) resistor thermally to an active device, such as a transistor, silicon controlled rectifier (xe2x80x9cSCRxe2x80x9d), thyristor, FET, etc., in order to control or protect the active device as a function of temperature of the PTC or NTC resistor. Several examples of power line protection devices and methods are described, for example, in the inventor""s commonly assigned, prior U.S. Pat. No. 6,072,681 issued on Jun. 6, 2000 to Cogan et al. and entitled: xe2x80x9cPower Line Protection Devices and Methodsxe2x80x9d, the disclosure thereof being incorporated herein in its entirety. Therein, a PPTC device is thermally coupled to an active element configured as a current switch via a common electrically insulative, thermally conductive substrate. If an overcurrent condition occurred at a load, the PPTC device trips, causing the current switch to open and disconnect power from the line/load. Computer monitoring and reset of the protection circuit is provided in some of the disclosed circuit configurations. Other examples of a PPTC device being thermally coupled to control an active device or circuit are given in commonly assigned U.S. Pat. No. 6,331,763 issued on Dec. 18, 2001 to Thomas et al. (including the present inventor), and entitled: xe2x80x9cDevices and Methods for Protection of Rechargeable Elementsxe2x80x9d, the disclosure thereof being incorporated herein by reference.
In particular the ""763 patent illustrates in FIGS. 45-47 a three-terminal device that includes FET regulator formed upon and thermally coupled to a PPTC substrate element. In these prior approaches, each circuit employed a single PPTC device in conjunction with an active element, and while that approach works well for certain applications, it has a drawback of high cost in circumstances where it is practical or necessary to control multiple lines simultaneously and with a single switch.
However, for some applications and environments, particularly harsh electrical environments such as motor vehicles, a hitherto unsolved need has remained to provide individual overcurrent protection to plural load lines and also to be able to disconnect all of the loads simultaneously in response to a single control signal. While separate circuits, such as fuses and switches or relays, may be connected in series to carry out these separate functions, a hitherto unsolved need has arisen for simultaneously providing these separate functions in a single device providing the advantages of multiple PPTC devices and characteristics.
A general object of the present invention is to provide an electrical device providing overcurrent protection to several low or medium power loads while providing simultaneous control of the loads as well as high voltage isolation between the power control element and the loads in a manner overcoming limitations and drawbacks of the prior art.
Another object of the present invention is to provide a method for protecting plural power loads by a single device which combines individual protection to each load and which may be actuated to disconnect simultaneously all of the loads in response to a control or a sensed condition.
An electrical device in accordance with principles and aspects of the present invention includes a package having an array of connections. A thermally conductive, electrically insulative substrate is provided in the package. Plural resistive elements are provided in the package. Each resistive element has a predetermined non-zero temperature coefficient, is connected to a different pair of connections of the array, and is in thermal contact with the substrate. At least one heating element is provided in the package and is in thermal contact with the substrate. While a variety of solid state passive and active heating elements may be successfully employed, one particularly preferred heating element enabling thermal autoregulation is a semiconductor device with a control electrode, such as a power field effect transistor (FET). The heating element has electrodes connected to associated ones of the array of connections and responds to a control current by heating the substrate and thereby indirectly heating the resistive elements, thereby to change resistance of the plurality of resistive elements. In one preferred aspect of the invention, the plurality of resistive elements comprises polymeric positive temperature coefficient (PPTC) resistors. In another preferred aspect of the present invention, the heating element comprises a semiconductor device having at least one junction, such as a power diode, a power field effect transistor, a power bipolar transistor, a power thyristor, or a power silicon controlled rectifier. In an alternative preferred aspect of the present invention, the substrate is provided by an integral electrode of the semiconductor device, and the device further includes an electrical insulator layer between the resistive elements and the substrate. In another alternative preferred aspect of the present invention, the substrate comprises a lead frame.
In one preferred arrangement of the present invention, an electrical device includes a package having an array of connections, a thermally conductive, electrically insulative substrate in the package, a plurality of polymeric positive temperature coefficient (PPTC) resistors in the package in thermal contact with the substrate wherein each PPTC resistor has a predetermined positive temperature coefficient and is connected to a different pair of connections of the array, and, at least one power field effect transistor in the package in thermal contact with the substrate, and having gate, source and drain electrodes connected to associated ones of the array of connections, for heating the substrate and for indirectly heating the PPTC resistors in response to a control current, thereby to trip the PPTC resistors from a low resistance state to a very high resistance state.
A method for providing separate electrical overload protection to a plurality of electrical loads and providing simultaneous disconnection of the loads in response to a disconnect control signal in accordance with principles of the present invention comprises the following steps:
providing a common heating substrate,
mounting or forming a plurality of polymeric positive temperature coefficient (PPTC) resistors to be in thermal contact with the common heating substrate,
connecting each PPTC resistor in series with one load of the plurality of electrical loads so that the PPTC resistor will trip to a high resistance state in response to an electrical overload at the one load,
providing a heating element such as a power FET in thermal contact with the common heating substrate, and applying the disconnect control signal to a control electrode of the power FET to heat the common heating substrate and thereby trip the plurality of PPTC resistors and simultaneously disconnect the electrical loads.
These and other objects, advantages, aspects and features of the present invention will be more fully understood and appreciated upon consideration of the detailed description of preferred embodiments presented in conjunction with the following drawings.